Carbon media for storage of hydrogen

ABSTRACT

This invention relates to a carbon media for storage of hydrogen characterized in that it comprises known and novel micro-domain materials and that it is produced by a one or two-step plasma process. In the one-step plasma process conventional carbon black or graphitic carbon black can be formed. In the two-step plasma process, a hydrocarbon feed material is sent through a plasma zone and becomes partly dehydrogenated in the first step to form polycyclic aromatic hydrocarbons (PAHs), and is then sent through second plasma zone to become completely dehydrogenated to form micro-domain graphitic materials in the second step. By micro-domain graphitic materials we mean fullerenes, carbon nanotubes, open conical carbon structures (also named micro-cones), flat graphitic sheets, or a mixture of two or all of these. The novel carbon material is open carbon micro-cones with total disclination degrees 60° and/or 120°, corresponding to cone angles of respectively 112.9° and/or 83.6°.

FIELD OF INVENTION

This invention relates to a carbon media for storage of hydrogen.

BACKGROUND OF THE INVENTION

There is currently an intense interest in carbon materials due to theirunique and novel properties. For instance, the carbon materials may beuseful to achieve high hydrogen energy storage, for use in purificationprocesses as well as for different applications within theelectronical/pharmaceutical sector. The properties are sensitive to themicro-structure of the carbon material, which can be varied by thenanostructure ordering (graphitisation level). The nanostructureordering spans from non-crystalline qualities such as conventionalcarbon black (furnace black, thermal black) to crystalline qualitiessuch as graphite and novel carbon materials with graphitic structures.The nanostructure ordering can be described in terms of the distancebetween the graphite layers, which will vary from 3.40 Å for a orderedcrystalline structure to 3.60 Å for non-crystalline materials.

The recent interest in carbon materials for use as a storage medium hasmainly been focused on novel materials with graphitic structures wherethe degree of graphitisation and the introduction of rings other thanhexagons in the network is of vital importance. Fullerenes are examplesof novel graphitic structures where the introduction of 12 pentagons inthe hexagonal network results in closed shells [1]. Carbon nanotubes isalso an example of such graphitic structures, but only three of fivepossible kinds have ever been synthesised [3, 4, 5].

Recent interest in fullerenes and nanotubes is amongst other connectedto their use in the field of hydrogen storage. Hence, for nanotubes ahydrogen storage of amazingly 75 wt % is reported [6]. If this is thecase, it will probably represent the break-through concerning apractical hydrogen storage system for use in the transportation sector.It is indicated that future fuel cell cars using this storage technologymay achieve a range of about 8000 km.

In the case of fullerenes, more than 7 wt % of reversibly added hydrogenis achieved [7, 8, 9]. Fullerenes has also been used in a solid phasemixture with inter-metallic compounds or metals to achieve high contentsof hydrogen, i.e. 24-26 H atoms per fullerene molecule [10].

Flat graphitic material formed of stacks of two-dimensional sheets hashigh surface area for adsorption of guest elements and compounds.However, in such materials the adsorption process is probably limited bydiffusion. The larger the graphitic domain, the slower the adsorptionwill be. Of potential interest would be highly graphitised materialswhere domains are small so is that the guest material would readilyreach all the graphitic micro domains by percolation through the bulkcarbon material. The accessibility to the micro-domains could be furtherenhanced if some or all the domains had topological disclination,preferably each domain having less or equal than 300° disclination toprovide cavities, or micro-pores, for the flow of guest material.

A common problem with the present methods for synthesizing thesegraphitic materials is the low production yield. The fullerenes are mostoften synthesized by [vaporising] vaporizing graphite electrodes viacarbon-arc discharges in a reduced inert gas atmosphere. There has beenreported a conversion rate into fullerenes of 10-15%, corresponding to ageneration rate of nearly 10 grams per hour [11].

The carbon-arc method is also the most frequently used method forproduction of carbon nanotubes. Nanotubes yields of about 60% of thecore material has been obtained at optimal conditions [2]. Still, theachieved production is in gram quantities.

Small unspecified amounts of open conical carbon structures are obtainedby resistively heating a carbon foil and further condensing the carbonvapour on a highly-oriented pyrolytic graphite surface [3, 4]. The coneangles produced by this method was approximately 19° [3], and 19° aswell as 60° [4].

Resistive heating of a carbon rod, with further deposition on coolersurfaces was used to produce cones with apparent cone angles ofapproximately 39° [5]. It can be shown from a continuous sheet ofgraphite that only five types of cones can be assembled, where eachdomain is uniquely defined by its topological disclination TD given bythe general formula:

TD=N×60 degrees, where N=0, 1, 2, 3, 4 or 5,

The structure of such graphitic domains can be grossly described asstacks of graphitic sheets with flat (N=0) or conical structures (N=1 to5). Hence, two of these, holding cone angles of 83.6° and 112.9°, hasnot been reported so far.

SUMMARY OF THE INVENTION

An object of this invention is to provide a carbon media for storage ofhydrogen. This object is achieved by a media characterised in that itcomprises known and novel crystalline or non-crystalline materials andthat it is produced by a two-step plasma process. By changing theprocess parameters of the plasma process, the nanostructure ordering ofthe carbon material can be varied in such a way that the desiredmicrostructure for optimum hydrogen storage is achieved. Thesemicrostructures may either be conventional carbon black graphitic carbonblack and/or novel carbon materials such as cones, fullerenes ornanotubes.

In the one-step plasma process conventional carbon black or graphiticcarbon black can be formed. A such process is described in for instanceEP 0 636 162. The resulting carbon material may have a surface area(BET) of 5-250m²/g and a dibutyl phtalate absorption (DBP) of 40-175 ml/100 g.

In the two-step plasma process, a hydrocarbon feed material is sentthrough a plasma zone and becomes partly dehydrogenated in the firststep to form polycyclic aromatic hydrocarbons (PAHs), and is then sentthrough second plasma zone to become completely dehydrogenated to formmicro-domain graphitic materials in the second step. By micro-domaingraphitic materials we mean fullerenes, carbon nanotubes, open conicalcarbon structures (also named micro-cones), Pat graphitic sheets, or amixture of two or all of these. The novel part of the carbon material isopen carbon micro-cones with total disclination degrees 60° and/or 120°,corresponding to cone angles of respectively 112.9° and/or 83.6°.

Another object of this invention is to provide a carbon media forstorage of hydrogen comprising known and novel micro-domain materials,characterised in that the media is produced in industrial scale withlarge yield rates of up to above 90% by a one or two-step plasmaprocess.

The invention also relates to use of the carbon media as a storage mediafor hydrogen.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic figure of the reactor and the surroundingequipment.

FIG. 2 shows a transmission electron microscope photograph of thesamples revealing the various types of open micro-conical carbons of theinvention.

FIG. 3 shows the projected angles for perfect graphitic cones, i.e.19.2°, 38.9°, 60°, 83.6° and 112.9°, which represents total disclinationof 300°, 240°, 180°, 120° and 60°, respectively. In addition a graphiticsheet, having a projected angle of 180° and a total disclination of 0°,is shown.

FIGS. 4A, 4B, 4C, 4D and 4E shows example of domains for each type ofdisclination 60°, 120°, 180°, 240° and 300°, respectively, present inthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The hydrogen storage capacity of the media is linked to the small sizeof the domains and the presence of various topographies in the material.These are useful for the incorporation of guest elements and compoundssuch as hydrogen. Also, the space between the domains will providemicro-pores for the flow of guest material so that it can reach eachdomain and the small size of the domains will allow rapid diffusion ofguest material in and out of each layer composing them.

The carbon media for storage of hydrogen in the present inventionconsists of known and novel micro-domain materials, such as conventionalcarbon black, fullerenes, carbon nanotubes, open conical carbonstructures (also named micro-cones), or flat graphitic sheets alone, ora mixture of two or all of these. The amount present of any of thecarbon structures, conventional carbon black, fullerenes, carbonnanotubes, micro-cones or flat graphitic sheets, in the media can beanywhere between 0 and 100 wt % based on the total mass of the hydrogenstorage media. Further, all possible mixtures of these structures can bepresent.

The novel carbon material is open carbon micro-cones with totaldisclination degrees 60° and/or 120°, corresponding to cone angles ofrespectively 112.9° and/or 83.6°. It can be shown that if a cone is madeup of an uninterrupted sheet of graphite, except at the open edge, onlyfive types are possible due to the symmetry of graphite. Thesecorresponds to a total disclination of 60°, 120°, 180°, 240° and 300°. Atotal disclination of 0° corresponds to a flat domain. FIG. 3 showsschematically the projected angles of these structures. Examples of eachof these types of domains are shown in FIGS. 4A, 4B, 4C, 4D and 4E. Itis important to notice that all the cones are closed in the apex. Thenovel carbon material of this invention comprises micro-domains ofgraphite of well-defined total disclinations TD (curvature), which havediscrete values given by the formula

TD=N×60 degrees, where N=0, 1, 2, 3, 4 or 5,

and corresponds to the effective number of pentagons necessary toproduce the particular total disclination.

The hydrogen storage media is produced in industrial scale bydecomposition of hydrocarbons into carbon and hydrogen in a one ortwo-step plasma based process. The plasma arc is formed in a plasmagenerator which consists of tubular electrodes, wherein the innerelectrode is supplied with electrical direct voltage with one polarityand wherein the external electrode is connected to the opposite polarityfrom a power supply. The plasma generator is installed in connectionwith a decomposition reactor wherein the reactor is designed as adefined heat insulated chamber with an outlet for end products. Theplasma gas is recycled from the process. Further description of thegeneral process and the equipment is described in the applicant'sEuropean patent EP 0 636 162.

The structure of the resulting carbon material will depend on thefollowing three process parameters: The hydrocarbon feed rate, theplasma gas enthalpy and the residence time. By varying these parametersthe resulting carbon material will either be available as conventionalcarbon black, as micro-domain materials or a mixture of both. In thefollowing we will describe the process parameters for optimising onmicro-domain materials. This illustrates the capability of the processto produce the carbon material which is most suited as a storage mediafor hydrogen.

Experimental results indicates that the total disclination nearly alwaysis determined in the nucleation stage. It is earlier found that theprobability of forming pentagons in the seed depends on the temperature[12]. Hence, by varying the process parameters, including but notlimited to increasing the reaction temperature, the number of pentagonsin the seed may increase. This may in turn result in formation onnanotubes or closed shells.

The hydrocarbons are introduced into the decomposition reactor in thevicinity of the plasma arc zone by use of a self-invented nozzle whichis aligning the hydrocarbon spray in the axial direction of the reactor.

Energy is supplied from the plasma arc to heat the plasma gas. Some ofthe energy from the arc will be used to heat the surrounding reactorwalls as well as the plasma generator itself. The resulting energycontent of the plasma gas (the plasma gas enthalpy) is sufficient toevaporate the hydrocarbons. The hydrocarbons starts a cracking andpolymerisation process, which results in the formation of PAHs. The PAHsare the basis of graphitic sheets forming the micro-domains. The plasmagas enthalpy is kept at such a level that the main fraction of thegaseous hydrocarbons does not reach pyrolysis temperatures at thespecified feedstock rate and residence time used. However, a smallfraction of the feedstock will inevitably achieve sufficient energyduring the residence time in the reactor to reach pyrolysis temperatureand is consequently converted to conventional carbon black. Thisfraction should be kept as low as possible.

The PAHs leaves the reactor along with the plasma gas and is once moreintroduced in the reactor as a part of the plasma gas. The plasma gasenters the energy intensive plasma arc zone, wherein the PAHs during afraction of a second are converted to graphitic seeds. These seeds aredictating the shape of the micro-domains. PAHs formed when introducingfresh feedstock will result in growth of the seeds to form graphiticmicro-domains.

Another alternative is to introduce the PAH containing plasma gas in asubsequent chamber or reactor which also is equipped with a plasmagenerator and an inlet for hydrocarbon feedstock to convert the PAHsinto graphitic micro-domains.

The feedstock feed rate for optimising on graphitic micro-domainmaterials is in the range of 50-150 kg/h in a reactor employed by theinventor, but is not limited by this range. Both lower and higherfeedstock feed rates might be used. The yield of the graphiticmicro-domain material is higher than 90% under optimal conditions. Takeninto account the feedstock feed rate utilised, industrial amounts ofmicro-domain carbon material is achieved. By further up-scaling thiswill result in a price which is on the same level as commercial carbonblack per unit weight of the material.

FIG. 1 shows a schematic drawing of the reactor. Further detailsconcerning the reactor and the surrounding equipment is described in theapplicant's European patent EP 0 636 162.

FIG. 2 shows a typical example of the content of the micro-domainmaterial. Each piece in the sample forms a single graphitic domain andthe alignment of the sheets in each domain is typically turbostratic, asdetermined from electron microscopy. The diameter of the domains istypically less than 5 micrometers and the thickness less than 100nanometers.

In the following, it will be demonstrated that by altering theconditions in the plasma reactor, it is possible to produce eitherconventional carbon black or micro-domain graphitic materials. Accordingto the present invention, both may be used as a hydrogen storage media.In Example 1, the process parameters are chosen in such a way thatconventional carbon black is formed at the first (and only) cycle ofhydrocarbons through the reactor. By varying the feedstock feed rate,the plasma gas enthalpy and the residence time, it is shown in Example 2that at the second cycle through the reactor, micro-domain graphiticmaterials can be produced from PAHs formed in the first cycle.

EXAMPLE 1

Heavy fuel oil was heated to 160° C. and introduced in the reactor byuse of the self-invented axial aligned nozzle at a feed rate of 67 kgper hour. The reactor pressure was kept at 2 bar. Hydrogen was employedas plasma gas, and the plasma gas feed rate was 350Nm³/h, while thegross power supply from the plasma generator was 620 kW. This resultedin a plasma gas enthalpy of 1.8 kWh/Nm3 H₂. The time elapsed from theatomised oil was introduced until the product left the reactor wasapproximately 0.23 seconds.

The resulting carbon black was traditional amorphous of N-7xx quality.The volatile content of the carbon black was measured to 0.6%.

EXAMPLE 2

In this example the oil feed rate, the hydrogen plasma gas enthalpy aswell as the residence time was tuned in such a direction that theevaporated hydrocarbons did not achieve pyrolysis temperature during thefirst cycle. The residence time of the hydrocarbons during the firstcycle through the reactor was minimised by increasing the oil and plasmagas feed rate.

Heavy fuel oil was heated to 160° C. and introduced in the reactor byuse of the self-invented axial aligned nozzle at a feed rate of 115 kgper hour. The reactor pressure was kept at 2 bar. The hydrogen plasmagas feed rate was 450 Nm³/h, while the gross power of supply from theplasma generator was 1005 kW. This resulted in plasma gas enthalpy of2.2 kWh/Nm³ H₂. The time elapsed from the oil was introduced until thePAHs left the reactor was approximately 0.16 seconds.

The resulting PAHs were reintroduced into the reactor in the plasma-arczone to produce a micro-domain graphitic material, with a yield higherthan 90%. The volatile content of the carbon material was measured to0.7%. All other process parameters were the same as for the first cycle.

Although in the example of the production of the hydrogen storage mediahas been described as a conversion of heavy oil to a micro-domaingraphitic material, it should be understood that the media can beproduced from conversion of all hydrocarbons, both liquid and gaseous.Also, the production may be performed as a batch or continuousproduction, with one or more plasma reactors in series etc. Concerningmicro-domain graphitic products where the PAHs formed in the firstdecomposition step is reintroduced into the same plasma reactor, themicro-domain graphitic materials formed in the second decomposition stepare of course separated from the PAH containing plasma gas by anyconventional suited means. This may be by filtering, cyclones etc.

Further, any gas that is inert and do not pollute the micro-domainproducts may be used as plasma gas, but hydrogen is specially suitedsince it is a product of the process. The plasma gas may be recycledback into the reactor, if desired. It is also possible to employ thepresent method by introducing additional hydrocarbons through inlets atthe sides of the decomposition reactor to control the temperature in thedecomposition zone and/or to increase the yield, see the applicant'sEuropean patent EP 0 636 162.

References

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5. R. Vincent, N. Burton, P. M. Lister and J. D. Wright, Inst. Phys,Conf. Ser., 138, p. 83, 1993.

6. Hydrogen & Fuel Cell Letter, vol. 7/No. 2, Feb. 1997.

7. R. M. Baum, Chem. Eng. News, 22, p. 8,1993.

8. Japanese Patent JP 27801 A2, Fullerene-based hydrogen storage media,18^(th) August 1994.

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What is claimed is:
 1. A carbon media for storage of hydrogen comprisingmicro-domain graphitic materials consisting of at least one of materialsselected from the group consisting of carbon nanotubes, fullerenes,carbon micro-cones and flat graphitic carbon sheets, wherein said mediaincludes micro-domain graphitic materials that have a graphitic stackingdirection and a domain size and which have been produced bydecomposition of hydrocarbons in a reaction chamber connected to aplasma generator in which the hydrocarbons are subjected to a firstdecomposition step and said plasma generator includes a plasma arc zone,where the hydrocarbons are fed into the decomposition chamber in thevicinity of the plasma arc zone and mixed with the plasma arc, and wherethe plasma arc zone is operated with adjustable process parameters andthe process parameters are adjusted in such a manner that thehydrocarbons do not reach pyrolysis temperature and are only partiallydecomposed to form polycyclic aromatic hydrocarbons (PAHs), -that thehydrocarbons in the form of PAHs are, after the first decompositionstep, mixed with a plasma arc and reintroduced as a part of a plasma gasinto a plasma arc zone in a decomposition chamber and subjected to asecond decomposition step, where the heat in the plasma arc zone causesthe PAHs to be converted into the micro-domain graphitic materials, thatthe domain size is smaller than 5 μm in diameter or length parallel to agraphitic stacking direction and having a thickness of less than 100 nmin the graphitic stacking direction.
 2. A media according to claim 1,wherein the media contains micro-domain graphitic materials in the rangefrom 0 to above 90 wt %.
 3. A media according to claim 2, wherein themedia contains more than 90 wt % micro-domain graphitic materials.
 4. Amedia according to any one of claim 1, wherein the media results fromdehydrogenation of heavy fuel oil into micro-domain graphitic materials.5. A carbon media for storage of hydrogen comprising micro-domaingraphitic materials, wherein said media contains open carbon micro-coneswith total disclination degrees of at least 60° and/or 120°,corresponding to cone angles of respectively at least 112.9° and/or83.6°.